There are different families of sushi:                Nigiri sushi: prepared rice ball (vinegar, sugar, salt) topped by a thin slice of raw fish or vegetable;        Maki sushi: prepared rice roll surrounded by dry seaweed and stuffed with a piece of raw fish or vegetable;        Guncan: prepared rice ball surrounded by dry seaweed and stuffed with vegetable, fish eggs or a vegetable- or fish-based preparation.        
Thus, all sushi is comprised of rice as well as at least one accompanying element.
Sushi is traditionally assembled just before its consumption, using sticky warm rice (between 20 and 30° C.) and one or more cool accompanying elements, i.e., at a temperature below 20° C. There are also foodstuffs that can be presented in a general manner like sushi as presented above. For example, canapés made from a slice of warm bread on which are deposited a layer of pâté, fruit or vegetables, fish eggs, partially cooked foie gras and the like.
When deep-frozen sushi or similar foodstuffs are proposed, the principal problem stems from the requirement of thawing in a manner such that the gustatory taste is a close as possible to those of products prepared in the traditional manner.
In fact, if the sushi or similar foodstuff is merely thawed with a microwave by simply placing the sushi or similar foodstuff in the oven, the principal element, i.e., the rice of the sushi or the bread, thawed in this manner is dried out and crumbles when it is grasped, on the one hand, and the accompanying element(s) begin(s) to be strongly heated and even to be cooked, on the other hand.
Already known in the prior art are specific devices for thawing sushi Thus, it is proposed in Japanese patent application No. 06/269253 to create a package formed by a container intended to receive a piece of frozen sushi, which container is closed with a cover. This container is made of a very thin metal foil, e.g., aluminum, to reflect the microwaves. The foil is connected to a synthetic resin support system except in a central part. The cover is made of the same material as the container, except for a central elliptical window allowing transmission of the microwaves. The side surfaces of the covers do not bear metal foil. Thus, the sushi placed inside such a container closed by means of such a cover is entirely surrounded by material that is strongly reflective of microwaves, with the exception of just above and just below the sushi where plastic material is found which allows passage of the microwaves.
The result is that the heat inside the package increases very quickly and very intensely, which has the effect of heating not only the rice but also the cool element(s) positioned around it or inside it, whereas this (these) element(s) should only be thawed and should conserve a temperature below 20° C.
It would therefore be advantageous to resolve the drawbacks of the prior art by providing a package that makes it possible to efficiently thaw one or more pieces of sushi or similar foodstuffs, i.e., to heat up the principal element (e.g., rice or bread) without heating up the accompanying element(s).